1. The Field of the Invention
The present invention is related to a system and process for inspecting and characterizing surfaces. More particularly, the present invention is related to a system and process for obtaining near real time, non-destructive inspection and characterization of thin films, particularly organic contaminants, utilizing optical filters and an array of detectors to analyze the spectral content of infrared light reflected off the surface.
2. Technical Background
A typical manufacturing process utilized in many applications is the bonding together of two materials. The criticality of the strength of the bond will vary depending on the particular application for which the bonded material is to be used.
A weak bond or area of debonding can be the source of stress risers which can result in further weakening of the bond. Eventually, these stress risers can lead to failure of the bond and can distort the geometry of the bonded material thereby adversely affecting the performance characteristics of the part.
When two materials are bonded together, contaminants on the surface of either of the materials can weaken the bond and, in some instances, cause areas of debonding. Organic materials such as greases, hydraulic fluids and mold release agents are a primary source of contamination of bonding surfaces in many production applications. Other contaminants which are frequently present in manufacturing environments include particulates such as sand or dust. Oil vapors are often present in environments where hydraulic systems and electric motors are operated. These vapors can condense on surfaces to be bonded. Even small levels of these contaminants, not visible to the human eye, can degrade bond strength.
The extent to which a surface can be cleaned prior to bonding and the method to be utilized in cleaning the surface vary according to the nature of the surface. For example, large-area, grit-blasted steel surfaces may be cleaned by a vapor degrease process. According to one such cleaning process, the part including the surface to be cleaned is suspended within a pit in the bottom of which boiling methylchloroform is located. The methylchloroform evaporates and condenses on the surface. As it runs off the surface, it dissolves any grease in its path. While this process works well in cleaning small amounts of grease from a grit-blasted metal surface, if there are areas of localized buildup of grease, not all of the grease may be removed by the cleaning process.
Using a solvent such as methylchloroform to clean a bonding surface may not be viable if the bonding surface is a porous material such as a phenolic. Phenolic materials will absorb virtually any type of cleaning solvent with which they come into contact. Contact with cleaning solvents can alter the surface chemistry and/or carry dissolved contaminants into the phenolic. In many applications involving phenolics, the surface properties of the phenolics must remain unchanged.
Presently, the preferred method of cleaning a phenolic material is to place it on the mill and machine a new surface, thereby removing the contaminated surface. However, this can only be done if the tolerances of the part permit a portion of the surface to be removed. Otherwise, a contaminated part may have to be replaced.
Because even small levels of contaminants, not visible to the human eye, can degrade bond strength, bonding surfaces must be inspected prior to bonding to ensure that there is no contamination, or that if there is contamination, it is within acceptable limits. Thus, an ideal surface inspection will reveal whether contamination exists, identify the contamination and measure the thickness of the contamination.
A crude method of conducting a surface inspection is to place some solvent on a wipe and stroke the surface with the wipe thereby transferring surface contaminants to the wipe. The wipe may then be analyzed using standard spectroscopy methods to verify the existence of contaminants on the wipe and determine their identity.
A principal obstacle to the successful use of this method is that it can only be used as a check method. It cannot be effectively used as an inspection method over a large-area surface. And, while the method may provide information about the existence of a contaminant and its identity, it cannot be used to provide any quantitative information concerning the thickness of the contamination. It is merely a qualitative method. Additionally, this method cannot be used with phenolic materials because the surface chemistry of the phenolics would be altered by bringing the surface into contact with a wipe permeated with solvent.
A more versatile surface inspection method is to conduct a visual inspection with the aid of an ultraviolet light. Some contaminants, particularly grease such as that used for rust protection, fluoresce under ultraviolet light. Thus, by visually inspecting the surface under ultraviolet light, any contaminants which fluoresce under the light can readily be detected.
The disadvantage of this method is that the method provides no information about the thickness of the contamination. Additionally, this method cannot be used to detect low levels of contamination as it is limited by what can be seen with the human eye. While attempts to instrument this method have been made, such an instrumented approach is prohibitively expensive for most applications. Also, depending on the nature of the potential contamination, contaminant detection may be frustrated as some contaminants do not fluoresce when illuminated with ultraviolet light.
Alternative inspection methods include an optically stimulated electron emission ("OSEE") method. The OSEE method is based on the photoelectric effect. By shining ultraviolet light on the surface to be inspected, electrons are emitted from the surface. By placing an electrode near the surface and raising the electrode to a predetermined voltage, an electric field is generated, drawing an electron current from the surface whose strength can be monitored. If there is contamination on the surface, the current is generally impeded.
A disadvantage to the OSEE method is that it is subject to many variables which are not relevant to the determination of contamination. Such variables may include air currents surrounding the device being tested, relative humidity and moisture on the surface. Also, the OSEE method only works effectively on metals. It is ineffective as a tool to inspect phenolic, rubber or oxidized metal surfaces.
Other optical methods have also been proposed. However, they are generally ineffective for the inspection of large-area surfaces because of the amount of time required to obtain and process a sufficient amount of data to provide meaningful and statistically reliable results. Additionally, the systems employed tend to be prohibitively expensive for use with most applications.
Contamination on a bonding surface is just one factor which can affect bond strength. Another parameter which can greatly affect the strength of a bond is the consistency of the thickness and the uniformity of chemical properties of the adhesive or other thin-film preparatory coatings applied to the bonding surface. And, of course, whether the adhesive completely covers the bonding surface also has an impact on the strength of the ultimate bond.
Generally, it is preferable to minimize the thickness of coatings applied to bonding surfaces. However, it is difficult to ascertain by non-contact methods the uniformity of a thin-layer coating or to readily ascertain whether the coating is applied to the entire surface.
The preparation of bonding surfaces is also important in obtaining an effective and consistent bond. Surface coatings or applications may be used to activate the surface to provide an enhanced chemical bond. When using such coatings or applications, the surface is prepared for bonding in a particular chemical state which must be consistent along the entire surface to maximize the strength of the bond. Of course, non-contact methods may not be employed to characterize the chemical state of a bonding surface.
For other applications it is also desirable to be able to characterize thin-layer coatings. For example, certain optical coatings, such as an anti-reflection coating, must have a consistent thickness to avoid adversely affecting the optics of the coated material. In the case of an anti-reflection coating, uniform thickness is prerequisite to causing the destructive interference of the reflected wave and thereby enabling the coating to effectively operate.
Thin films are also employed in some applications to obtain a particular color or appearance. In many cases, obtaining the desired aesthetic effect is dependent upon the consistency of the thickness and optical properties of the film.
From the foregoing it will be appreciated that it would be an advancement in the art to provide a system and process for the inspection of surfaces which would detect the presence of contamination, including low-level contamination which may not be detectible with prior-art visual inspection methods.
It would be a further advancement in the art to provide such a system and process which could be utilized in determining the identity of the contaminant.
It would be an additional advancement in the art to provide a surface inspection system and process which could measure the thickness of a thin film, thereby enabling determination of whether the level of contamination on a contaminated surface is within permissible limits.
Indeed, it would also be an advancement in the art if such a surface inspection system and process could work effectively to inspect and characterize thin films on a variety of surfaces and with different levels of roughness, including metal, phenolic and rubber surfaces.
It would be yet a further advancement in the art to provide such a system and process that could work efficiently and effectively in inspecting large surface areas and at a cost which would render the system suitable for use on a variety of applications.
It would be an additional advancement in the art if such a system and process for inspecting thin films could be utilized to characterize chemical and optical properties of thin films.
Such a surface inspection system and process is disclosed and claimed herein.